Thermal Effects

Thermal Effects on Trophic Community Function

funded by Electric Power Research Institute and Carolina Power and Light

Project Description

        Extensive biological monitoring data have been collected for ten years about the aquatic community in a black-water reservoir in the southeastern United States. The reservoir community is typical of many similar water bodies in the region. The most abundant fish species is bluegill Lepomis macrochirus. The dominant predator in the reservoir is largemouth bass Micropterus salmoides. Is the decade-long record of ecological data sufficient to permit the determination of the potential long-term population-level consequences of relaxing environmental regulations on thermal discharges from a power plant?

        Despite the quality of the biological sampling data, there will always be some uncertainty in predicting the true risk of future population declines, due to inherent variability of climate, reservoir inflow, nutrient cycling, etc. associated with population response. Nevertheless, it may be possible to predict the relative effect of a marginal change in environmental conditions compared to the background risk for the populations of interest.

        The analysis included the patterns of temperature fluctuations and fish, zooplankton and phytoplankton abundances over the last ten years. The reason for including the lower food chain levels is that thermal impacts may have indirect effects through trophic interactions that are more pronounced than the direct effects of temperature on the fish species.

        Even using worst case assumptions about how increases in discharge temperature would translate into increases in water temperature, simulation results suggest that relaxing thermal discharge limits by a few degrees is likely to have little or no effect on the risk that largemouth bass will experience population declines of any magnitude. The effect on bluegill, on the other hand, appears to depend on when during the year the temperature increases occur. If the increases affect maximum summer temperatures, there may be a beneficial effect so that the risk of bluegill population declines are actually reduced. However, if the increases affect maximum springtime temperatures during the spawning period, there may be an increase in the risk of population decline, although this increased risk for bluegill has virtually no impact on its predator, largemouth bass. The available data and the simulations based on them offer no suggestion of any adverse indirect ecological effects on either species by temperature impacts on their food supply lower on the food chain.

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